12meters
meters
You would probably measure a keyboard in centimeters (cm).
Meters, centimeters, or millimeters. Take your pick.
cm
That depends on the individual desk, doesn't it - but you don't mix units quite like that in the metric system. You would quote the dimensions of a piece of furniture in either metres to 2 or 3 decimal places, or whole numbers of millimetres. E.g. 1.25m OR 1255mm. Not both. Shops, and UK schools, tend to use the centimetre although it is NOT a "preferred" division of the metre, and then you'd measure a desk in only cm, not m and cm.
2
Usually ... a desk top will be quicker because speed equals heat and a desk top can have bigger heat sinks. Note that there will be exceptions, but in the long run the desk top will stay ahead.
Typically, you measure a desk height with feet and a desk width with feet also. But, it depends on the size.
desktop
I prefer to use millimetres or if the measurement is over 1 metre I would say 1.25 or 12 hundred and 50. So a desk might be 900x500 x750 high
In SI units, use centimeters or meters. In non-SI units, use inches or feet.
Scale and inch-tape can be used to measure desk. It is 1-2 m long.